Para-type wholly aromatic copolyamide fibers made of an aromatic dicarboxylic acid component and an aromatic diamine component as main components are characterized by high strength, high tensile elastic modulus, low creep, high heat resistance, high chemical resistance, etc., and thus have been conventionally used for various industrial material applications, including reinforcing material applications for resin, rubber, and the like, ropes, etc. These industrial materials also have found increasing applications in severer environments such as high-temperature environments, and the physical properties and characteristics of para-type wholly aromatic copolyamide fibers are expected to be further improved.
Such a para-type wholly aromatic copolyamide fiber is generally produced by a so-called “wet spinning method” or “half-dry half-wet spinning method.” That is, an optically isotropic solution for spinning is extruded through a spinneret, and, after once being passed through air or an inert gas, or directly without being passed, brought into contact with a coagulation liquid in a coagulation bath, followed by steps of washing with water, drying, drawing, etc., to give a final fiber.
Incidentally, generally, a para-type wholly aromatic copolyamide fiber is produced using an optically isotropic solution for spinning. Therefore, its high mechanical properties, such as high strength and high tensile elastic modulus, are not developed until the fiber is drawn.
However, because the molecular structure of a para-type wholly aromatic copolyamide polymer is extremely rigid, it has been necessary to, prior to drawing, plasticize the fiber to some extent at a high temperature close to the decomposition temperature of the polymer or by swelling in a solvent in which the polymer is soluble. Therefore, it has been difficult to perform drawing under high tension, and, as a result, it has been impossible to sufficiently increase the molecular orientation, leading to the problem that the tensile elastic modulus is slightly inferior as compared with a high-performance fiber produced using an optically anisotropic solution for spinning, such as a para-type wholly aromatic polyamide fiber, a typical example thereof being paraphenylene terephthalamide (hereinafter, PPTA).
Accordingly, various studies have been made to improve the tensile elastic modulus of a para-type wholly aromatic copolyamide fiber. For example, Patent Documents 1 to 3 report a method in which a para-type wholly aromatic copolyamide fiber produced by the “wet spinning method” or “half-dry half-wet spinning method” is subjected to a tensioning heat treatment under specific conditions.
However, according these methods, because the fiber is heat-treated under extremely high tension, some of the fiber-forming single yarns break. This not only significantly decreases the quality of the fiber, but also reduces the number of single yarns effective for the development of tensile elastic modulus. Accordingly, as a result, the tensile elastic modulus as a fiber bundle has not been much improved, and a sufficiently satisfactory tensile elastic modulus has not yet been achieved at present.